1. Field of the Invention
The present invention generally relates to surface acoustic wave devices implemented as resonators or bandpass filters, and a method of producing the same. More particularly, the present invention relates to a surface acoustic wave device using a relatively heavy electrode material so that shear-horizontal surface waves can be utilized, and to a method of producing the same.
2. Description of the Related Art
A surface acoustic wave (SAW) device is widely used as a resonator or a bandpass filter. A typical surface acoustic wave device has a piezoelectric substrate, and electrodes of interdigital transducers (IDTs) and reflectors disposed on the piezoelectric substrate, which are made of an electrode material mainly including Al.
As known in the art, a surface acoustic wave device having electrodes made of tungsten (W) or tantalum (Ta), which is relatively heavy, could utilize shear-horizontal (SH) surface acoustic waves.
However, in such a surface acoustic device using a relatively heavy electrode material, variations in film thickness or line width are inevitable when the electrodes of IDTs and reflectors are formed. The surface acoustic wave device using a relatively heavy electrode material would cause considerable variations in the acoustic velocity distribution, or the frequency distribution, of surface acoustic waves because the electrode material is heavier than the electrode material of a typical surface acoustic wave device which mainly includes Al. As a result, significant and critical ripples are exhibited on the group delay time characteristic within a bandpass area, as indicated by an arrow A in FIG. 10.
FIG. 10 depicts the attenuation frequency characteristic versus the group delay time characteristic of a longitudinally coupled resonator SAW filter having two IDTs with a center frequency of 225 MHz. The longitudinally coupled resonator SAW filter has a piezoelectric substrate using a quartz substrate (37xc2x0 rotated Y-cut plate), and IDTs and reflectors which are made of Ta.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device using a relatively heavy electrode material such as Ta so that SH surface acoustic waves can be utilized, wherein the acoustic velocity distribution, or the frequency distribution, of the surface acoustic waves in the extending direction of the electrode fingers is minimized, thereby suppressing ripples within a bandpass area, and to provide a method of producing such a surface acoustic wave device.
According to a preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate, and at least one interdigital transducer having a plurality of electrode fingers extending substantially parallel to each other. The interdigital transducer is disposed on the piezoelectric substrate, and is made of a metal heavier than Al or an alloy heavier than Al. The acoustic velocity distribution of surface acoustic waves in the extending direction of the electrode fingers of the interdigital transducer is not greater than about 276 ppm.
Preferably, the surface acoustic wave device further includes a pair of reflectors each having a plurality of electrode fingers, the pair of reflectors being disposed on the piezoelectric substrate, wherein the acoustic velocity distribution of the surface waves in the extending direction of the electrode fingers of the pair of reflectors is not greater than about 276 ppm.
The at least one interdigital transducer may include first and second interdigital transducers arranged side-by-side in the propagating direction of the surface acoustic waves. The pair of reflectors are positioned at both sides of the location where the first and second interdigital transducers are positioned in the propagating direction of surface waves. The first and second interdigital transducers, and the pair of reflectors define a longitudinally coupled resonator filter.
Preferably, the surface waves are SH surface waves such as Love waves.
The acoustic velocity distribution of the surface acoustic waves in the piezoelectric substrate on which the at least one interdigital transducer is disposed may incline substantially perpendicularly to the extending direction of the electrode fingers of the at least one interdigital transducer.
Another preferred embodiment of the present invention provides a method of producing a surface acoustic wave device including a piezoelectric substrate, and at least one interdigital transducer having a plurality of electrode fingers extending substantially parallel to each other, the interdigital transducer being disposed on the piezoelectric substrate and made of a metal heavier than Al or an alloy heavier than Al, wherein the acoustic velocity distribution of surface waves in the extending direction of the electrode fingers of the interdigital transducer is not greater than about 276 ppm. The method includes the steps of preparing a wafer having a plurality of surface acoustic wave devices provided thereon, measuring the acoustic velocity distribution of the surface acoustic waves in the extending direction of the electrode fingers in each of the surface acoustic wave devices disposed on the wafer, and cutting out of the wafer the surface acoustic wave devices in which the acoustic velocity distribution of the surface waves is not greater than about 276 ppm.
In still another preferred embodiment of the present invention, there is provided a method of producing a surface acoustic wave device, the surface acoustic wave device including a piezoelectric substrate, and at least one interdigital transducer having a plurality of electrode fingers extending substantially parallel to each other, the interdigital transducer being disposed on the piezoelectric substrate and made of a metal heavier than Al or an alloy heavier than Al, wherein the acoustic velocity distribution of surface acoustic waves in the extending direction of the electrode fingers of the interdigital transducer is not greater than about 276 ppm. The method includes the steps of depositing a metal film on the piezoelectric substrate to form the interdigital transducer thereon, and patterning the metal film via photolithography. The depositing step and the patterning step are performed such that the effect of the film thickness profile in the extending direction of the electrode fingers on the acoustic velocity distribution is cancelled out by the effect of the film width profile in the extending direction of the electrode fingers on the acoustic velocity distribution.
A surface acoustic wave device according to another preferred embodiment of the present invention has a piezoelectric substrate, and at least one IDT disposed thereon which is made of a metal or an alloy that is heavier than Al. Since the acoustic velocity distribution of surface acoustic waves in the extending direction of the electrode fingers of the IDT is not greater than about 276 ppm, ripples which may be caused by the relatively heavy electrode material within a bandpass area can be effectively suppressed. Therefore, the surface acoustic wave device provides a superior frequency characteristic in which the ripples are reduced within the bandpass area.
A surface acoustic wave device having reflectors, in which the acoustic velocity distribution of the surface acoustic waves in the extending direction of the electrode fingers of the reflectors is not greater than about 276 ppm, also effectively suppresses ripples within the bandpass area.
The surface acoustic wave device according to preferred embodiments of the present invention is not limited to a longitudinally coupled resonator filter. The present invention may be applied to a variety of surface acoustic wave devices, and a longitudinally coupled resonator filter in which ripples are reduced within a bandpass area may be achieved according to preferred embodiments of the present invention.
Typically, a surface acoustic wave device which utilizes SH surface waves has electrodes made of a metal or an alloy that is heavier than Al, causing ripples due to the heavier electrode material. The present invention enables the ripples to be effectively minimized. The surface acoustic wave device that utilizes SH surface waves, such as Love waves, provides a superior frequency characteristic.
According to preferred embodiments of the present invention, the acoustic velocity distribution of the surface acoustic waves in the piezoelectric substrate on which IDTs are provided inclines substantially perpendicularly to the extending direction of the electrode fingers of the IDTs. This makes it easy to set the acoustic velocity distribution of the surface waves in the extending direction of the electrode fingers of the IDTs to be not greater than about 276 ppm.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.